Modifications to Prefab Homes

Looking at getting a new prefab factory built home for our lot but notice we will have to make a number of changes to fit our style. Question, does it start getting expensive when you start changing the production line plan of a prefab home, like moving rooms around or changing sqrft, etc.? Or are just paying extra on the hrs it takes to change the CAD plan prior to construction, (plus extra materials if needed in the changes)? -thanks